The invention relates to a level control of a road vehicle, in particular, to the adjustment of a vehicle level within a given range by detecting the relative height between the axle and the frame of a carrosserie by means of a level detector and controlling a hydraulic pressure supplied to a suspension system in accordance with the detected height.
An example of the level adjustment of the kind described is disclosed in U.S. Pat. No. 4,105,216 (issued Aug. 8, 1978, Class 280), for example, in which a level detector detects a range of vehicle level, and the detected signal is processed in a signal processing circuit to derive a signal which energizes a levelling drive system. The signal is applied to a level control circuit which causes a hydraulic pressure supplied to a suspension system to be reduced if the vehicle level is determined to be "high" and causes the hydraulic pressure to be increased if the vehicle level is determined to be "low". In order to prevent a repetitive operation to reduce or increase the hydraulic pressure which would occur as the detected vehicle level oscillates adjacent to the boundary between "medium" and "high" or between "medium" and "low" region, the level control circuit includes a set of leading end delay circuits in each of "high" and "low" level signal processing loops.
The levelling drive system is energized to lower or raise the vehicle height at a given time interval after the detection of the vehicle height in either high or low region, respectively. If the vehicle level is determined to be in the medium region, the levelling drive system is immediately deenergized. Therefore, there is a tendency for the vehicle level to be settled in the medium region adjacent to the boundary with the high or the low region, resulting in a large deviation which is obtained in the settled level of the vehicle. In addition, the frequency of the energization or deenergization of the levelling drive system increases.